1. Field of the Invention
This invention relates to apparatus and methods for testing electrical connections between conductors on an insulating body by applying an electron beam to the surface of the device under test.
2. Related Art
U.S. Pat. Nos. 4,578,279; 4,417,203; and 4,415,851 (commonly assigned to the assignee of the instant application) all relate to use of E-beam testing of substrates.
U.S. Pat. No. 4,415,851 of Langner et al for "System for Contactless Testing of Multi-layer Ceramics" and U.S. Pat. No. 4,417,203 of Pfeiffer et al "System for Contactless Electrical Property Testing of Multi-layer Ceramics" disclose systems for testing of networks including both top to bottom and top to top connections. The systems include two E-beam flood guns and one scanning beam gun. The scanning beam is arranged to scan the top surface of the specimen under test. One flood gun irradiates the bottom surface for top to bottom testing. The other flood gun irradiates the top for testing top to top connections.
U.S. Pat. No. 4,578,279 of Zingher for "Inspection of Multilayer Ceramic Circuit Modules by Electrical Inspection of Unfired Green Sheets" describes employing electrical testing techniques including E-beam testing techniques.
W. H. Bruenger, F. J. Hohn, D. P. Kern, P. J. Coane, and T. H. P. Chang, "Electron Energy Analyzer for Applications in Large Scan Field for Electron Beam Testing", Proceedings of the Symposium on Electron and Ion Beam Science and Technology, Tenth International Conference 1982, The Electrochemical Society, Proceedings Vol. 83-2, pp. 159-169 (1983) shows in FIG. 4 a four channel retarding field analyzer with successive extraction grid, "retard" grid and collector grid at a 45 degree angle to the vertical with respect to the target of the E-beam, with a sensor and photomultiplier tube behind the collector grid.
P. J. Fentem and A. Gopinath "Voltage Contrast Linearization with a Hemispherical Retarding Analyzer", Journal of Physics E: Scientific Instruments pp.930-933 (1974) Vol. 7, shows on page 931 a pair of hemispherical grids in both FIGS. 1a and 1b with grid B being a retarding grid. The article states, ". . . two hemispherical grids, which form the retarding analyzer, with the specimen at the centre." FIG. 1b shows a scintillator cage beyond the retarding grid B. With respect to the scintillator cage, the article states "Final collection was achieved . . . by placing a conventional scintillator cage detector close to the grids." It is stated in Fentem, et al with respect to FIG. 1a that grid A "is held at +60 V to ensure that secondary electrons emitted from the specimen are attracted outwards, although this results in the grid collecting some electrons. This grid should be at least, say, 15 V above the most positive potential applied to the retarding grid [B] because a rise in collected current at the solid hemisphere is observed when the potential of the retarding grid approaches that of the inner grid." From the point of view of this invention, Fentem, et al has drawbacks. Firstly, secondary electrons are caused to return to the sample. Secondly, there is no peak in the voltage contrast characteristic, the desirability of which is discussed later. Thirdly, the system is suitable only for relatively small samples.
S. D. Golladay, H. C. Pfeiffer, and M. A. Sturans, "Stabilizer Grid for Contrast Enhancement in Contactless Testing of MLC Modules", IBM Technical Disclosure Bulletin Vol 25, No. 12, May 1983, pp. 6621-6623) shows testing with E-beams using a negatively biased stabilizer grid between a flood gun and a sample.
U.S. Pat. application Ser. No. 925,764, filed Oct. 30, 1986, now U.S. Pat. No. 4,843,330, of Golladay, Hohn and Pfeiffer for "Electron Beam Contactless Testing System with Grid Bias Switching" describes an electron beam testing system wherein a grid is located above the device under test. The grid is employed both to assist charging and to extract secondary electrons from the device under test by switching the bias voltage applied to the grid.
3. Technical Problems
In accordance with the process employed by the method and apparatus of this invention, it is required that a relatively high energy beam should charge the circuit to be tested and then that a lower energy beam should be applied which will measure the voltage existing on each point on the network.
For the practical application of this test method novel apparatus and methods are required beyond that described in the related art. In particular, the following are required:
1) a suitable voltage contrast detector, PA0 2) a method to control the charging process, and PA0 3) methods to test capacitively coupled networks. PA0 1. Good contrast between charged and uncharged nets, i.e. good voltage contrast, PA0 2. Physical compatibility with a large scan field, PA0 3. Uniform contrast over a large substrate, PA0 4. Non-return of low energy secondaries to the sample surface of the specimen under test. The return of such electrons can alter the electrical potential on pads or vias adjacent to the pads or vias which are being tested. This can produce a false indication of detection of defects in the specimen. PA0 a) means for directing an electron beam at a sample having a conductive element thereon, PA0 b) charging means for electrically charging the conductive element on the sample, PA0 c) grid means for developing a positive charge for accelerating secondary electrons from the sample including PA0 d) deflection means located above the sample comprising negatively charged elements for generating an electric field above the sample adjacent to the lateral grid means for deflecting secondary electrons generated on the sample laterally away from the beam and away from the sample and towards the lateral grid means. PA0 e) means for deflecting lower energy ones of the secondary electrons, and PA0 f) means for trapping the lower energy ones of the secondary electrons from the sample to protect the sample from the lower energy electrons. PA0 e) attraction means for attracting electrons located laterally with respect to the sample and below the lateral grid means, the attraction means being positively charged to capture low energy secondary electrons. PA0 e) detector means for detecting the charge level during charging of the sample by the charging beam means, the detector having an output, PA0 f) blanking means for blanking the charging beam means to arrest the charging of the sample in response to an input thereto, the blanking means having an input, PA0 g) peak detector means for detecting a peak voltage, and PA0 h) comparator means for actuating the blanking means to blank the charging beam subsequent to detection of a peak voltage, the comparator means having an output connected to the input of the blanking means and the comparator means having inputs connected to the output of the peak detector means and the output of the detector means. PA0 a) an electron beam directed at a sample having a conductive element thereon, PA0 b) charging means for electrically charging the conductive element on the sample, PA0 c) grid means for developing a positive charge for attracting secondary electrons from the sample located PA0 d) detector means for detecting the charge level during charging of the sample by the charging beam means, the detector having an output, PA0 e) blanking means for blanking the charging beam means to arrest the charging of the sample in response to an input thereto, the blanking means having an input, PA0 f) peak detector means for detecting a peak voltage, and PA0 g) comparator means for actuating the blanking means to blank the charging beam subsequent to detection of a peak voltage, the comparator means having an output connected to the input of the blanking means and the comparator means having inputs connected to the output of the peak detector means and the output of the detector means. PA0 a) applying an unfocused flood electron beam with a low current to a broad surface of the body, simultaneously applying a different focused probe electron beam having an energy predetermined to provide a charge of opposite polarity from the flood beam to other areas of the body to be probed, PA0 b) generating an electron beam to cause secondary electron emission from the conductors; and PA0 c) detecting the presence of connections not at a given potential, whereby it is assured that the effects of capacitive coupling are minimized in producing a charge at any given point. PA0 A. performing a complete intra group test on said connections in group one to find all opens on networks within group one and all shorts between nets within group one, the intra group test being performed as follows: PA0 B. perform a complete intra group test from steps (1) to (5) on group two, PA0 C. Perform a complete intra group test on group three, PA0 D. this pattern in steps B and C is repeated with each group from group one to group N where N is an integer,
For the purposes of electrical testing of substrates, a "suitable" voltage contrast detector must have the properties as follows: